FormData: append() method

Note: This feature is available in Web Workers.

The append() method of the FormData interface appends a new value onto an existing key inside a FormData object, or adds the key if it does not already exist.

The difference between set() and append() is that if the specified key already exists, set() will overwrite all existing values with the new one, whereas append() will append the new value onto the end of the existing set of values.


append(name, value)
append(name, value, filename)



The name of the field whose data is contained in value.


The field's value. This can be a string or Blob (including subclasses such as File). If none of these are specified the value is converted to a string.

filename Optional

The filename reported to the server (a string), when a Blob or File is passed as the second parameter. The default filename for Blob objects is "blob". The default filename for File objects is the file's filename.

Note: If you specify a Blob as the data to append to the FormData object, the filename that will be reported to the server in the "Content-Disposition" header used to vary from browser to browser.

Return value

None (undefined).


formData.append("username", "Chris");

When the value is a Blob (or a File), you can specify its name with the filename parameter:

formData.append("userpic", myFileInput.files[0], "chris.jpg");

As with regular form data, you can append multiple values with the same name:

formData.append("userpic", myFileInput.files[0], "chris1.jpg");
formData.append("userpic", myFileInput.files[1], "chris2.jpg");

If the value is not a string or a Blob, append() will convert it to a string automatically:

formData.append("name", true);
formData.append("name", 72);
formData.getAll("name"); // ["true", "72"]


XMLHttpRequest Standard
# dom-formdata-append

Browser compatibility

BCD tables only load in the browser

See also